The development of optical fibre switching components is vital to the continued growth of global information systems. Single-stage matrix switches operating independently of the optical bit-rate and modulation formats, capable of reconfigurably interconnecting N optical inputs to M optical outputs (where N and M are generally, but not necessarily the same number), are particularly desirable.
Some prior art switches are limited in functional size to less than 64×64. Others suffer from relatively poor noise performance.
One useful known configuration is described later herein with respect to FIG. 15. This switch, which uses static holograms provides a static optical switch in which the input signals are “hard-wired” to specific outputs. Adapting the device of FIG. 15 to use reconfigurable holograms as elements for deflecting optical beams in free-space between arrays of optical inputs and optical outputs provides a reconfigurable switch by means of displaying hologram patterns on a spatial light modulator.
There are, however, some practical design problems associated with the migration from a static optical shuffle to a reconfigurable switch.
One problem which has failed to be successfully addressed is that of crosstalk, and another at least partly allied problem is that of insertion losses. Crosstalk occurs when light that was intended to follow one path instead has a component that follows another path. The insertion loss issues are linked to imperfections in the hologram displaying device and to the use of microlenses, which are difficult and expensive to produce, as well as being of questionable accuracy and poor reproducibility. Both of these defects lead to crosstalk. The hologram displaying devices used in reconfigurable hologram switches have been found to be less than perfect, in that they allow beams of direct light to pass through when a deviation was required instead. Clearly the direct component (referred to herein also as “zero order” light) gives rise to crosstalk.
A third problem is making the switch polarisation insensitive, since the polarisation of the light passing through an optical network, and especially through optical fibres fluctuates, for example with time. It has been established that ferroelectric liquid crystal SLMs can be made to operate in a polarisation insensitive manner.
These effects can be addressed by selection of the holograms. However state of the art techniques for production of holograms are not adequate for the required performance.
It is a primary object of the present invention to provide an optical switch using reconfigurable hologram devices which at least partly overcomes the problems of the state of the art.